Nitroaromatic compounds, from synthesis to biodegradation

Abstract

Nitroaromatic compounds are relatively rare in nature and have been introduced into the environment mainly by human activities. This important class of industrial chemicals is widely used in the synthesis of many diverse products, including dyes, polymers, pesticides, and explosives. Unfortunately, their extensive use has led to environmental contamination of soil and groundwater. The nitro group, which provides chemical and functional diversity in these molecules, also contributes to the recalcitrance of these compounds to biodegradation. The electron-withdrawing nature of the nitro group, in concert with the stability of the benzene ring, makes nitroaromatic compounds resistant to oxidative degradation. Recalcitrance is further compounded by their acute toxicity, mutagenicity, and easy reduction into carcinogenic aromatic amines. Nitroaromatic compounds are hazardous to human health and are registered on the U.S. Environmental Protection Agency's list of priority pollutants for environmental remediation. Although the majority of these compounds are synthetic in nature, microorganisms in contaminated environments have rapidly adapted to their presence by evolving new biodegradation pathways that take advantage of them as sources of carbon, nitrogen, and energy. This review provides an overview of the synthesis of both man-made and biogenic nitroaromatic compounds, the bacteria that have been identified to grow on and completely mineralize nitroaromatic compounds, and the pathways that are present in these strains. The possible evolutionary origins of the newly evolved pathways are also discussed.

FIG. 1.

Nitroaromatic explosives.

FIG. 2.

Pesticides synthesized from nitrophenols.

FIG. 3.

Nitroaromatic antibiotics produced by bacteria of the genus Streptomyces.

FIG. 4.

Rufomycins and ilamycins produced by bacteria of the genus Streptomyces.

FIG. 5.

Nitroaromatic siderophores produced by bacteria of the genus Streptomyces. Ferroverdins are similar to viridomycins but have three variously substituted p-vinylphenyl-3-nitroso-4-hydroxybenzoate groups bound to the Fe²⁺ (R groups are shown). Three p-vinylphenyl-3-nitroso-4-hydroxybenzoate groups are bound in ferroverdin A. In contrast, ferroverdins B and C are composed of two molecules of p-vinylphenyl-3-nitroso-4-hydroxybenzoate, with a hydroxy (ferroverdin B) or carboxylic acid (ferroverdin C) p-vinylphenyl-3-nitroso-4-hydroxybenzoate functional group as the third group.

FIG. 6.

Nitroaromatic phenylpyrrole antibiotics.

FIG. 7.

Nitroaromatic metabolites produced by plants and fungi.

FIG. 8.

Nitroaromatic signaling molecules.

FIG. 9.

Nitrobenzoate degradation pathways.

FIG. 10.

Nitrophenol degradation pathways.

FIG. 11.

Methyl parathion, 4-nitroanisole, and 4-nitrophenol degradation pathways.

FIG. 12.

Di- and trinitrophenol degradation pathways. (A) 2,6-Dinitrophenol degradation pathway; (B) 2,4-dinitrophenol and picric acid (2,4,6-trinitrophenol) degradation pathways.

FIG. 13.

Nitrobenzene degradation pathways.

FIG. 14.

Pathways for 2-nitrotoluene and 3-nitrotoluene degradation.

FIG. 15.

4-Nitrotoluene degradation pathways.

FIG. 16.

Degradation pathways for 2,4-dinitrotoluene and 2,6-dinitrotoluene.

FIG. 17.

Chloronitrobenzene degradation pathways. (A) Pathways found in natural isolates. (B) Engineered pathway in Ralstonia sp. JS705. (Panel B adapted from reference with permission of Blackwell Publishing Ltd.)

FIG. 18.

Degradation pathways for 3-nitrotyrosine and 3-nitrotyramine.

FIG. 19.

Dioxygenase gene clusters in naphthalene- and nitroarene-degrading bacteria. Numbers in parentheses denote amino acid identities shared with the corresponding protein components of naphthalene dioxygenase in Ralstonia sp. U2.

FIG. 20.

The nitroarene dioxygenases have ancestral roots in the naphthalene dioxygenase enzyme system. Electrons are transferred from NAD(P)H through reductase and ferredoxin proteins to the catalytic (α) subunit of the dioxygenase to allow catalysis to occur.